1. Field of the Invention
This invention relates to spot welding and, more particularly, to a method of controlling various weld conditions during spot welding, such as welding current and resistance welding time, depending upon the degree of wear of the electrode tip.
2. Description of the Prior Art
In a spot welder, as the welding working time increases, the point of the electrode tip gradually swells and its end shape is slowly deformed to increase the area of the tip point due to the strong pressing force, heating, etc. at the time of welding. Weldability is influenced directly by such deformation.
In view of this circumstance, in order to provide a good welding quality it is necessary in general to dress the point of the electrode tip or to periodically replace the tip with a new one to elongate the interval between maintenance work, in the prior art, a compensation operation was performed to compensate for the change in shape of the point of the electrode tip, through the use of a so-called current stepper control system, wherein, in accordance with the estimated degree of progressive wear of the electrode tip, the welding current is discretely increased each time a given number of welding operations is performed. Alternatively, a linear current stepper control system has been used wherein the welding current is progressively increased each time of welding.
In the present-day multiproduct-mixed production line handling galvanized sheet iron, polished soft steel plate, etc. and using of a number of welding robots, however, it is necessary to perform welding of various different materials. At the time of spot-welding galvanized sheet iron, a large current is required, and staining or wear of the electrode tips is heavier in comparison with the case of processing bare steel plate; thus the interval of tip shaping and exchange becomes correspondingly short. From the view point of productivity, such time-consuming incidental work becomes non-ignorable, and the compensation function attained via the current stepper control system or the linear current stepper control system was not satisfactorily under such circumstances. The conventional systems caused a number of problems (described below); thus other countermeasures were sought out.
Describing in detail the conventional techniques, neither the current stepper control system or the linear current stepper control system can calculate the proper rate of current increase and supply a corresponding welding power each time a different metal is subjected to welding in a multiproduct-mixed production line. Conventionally, only the number of instances of welding (the welding count) is employed as a parameter for compensation, and is used to provide a measure of the estimated welding quality. That is, the conventional systems do not suggest the control of the current increase in relation to the actual degree of wear of the electrode tip, and the rate of current increase does not necessarily agree with or correspond to the actual degree of wear of the electrode tip.
Therefore, in the prior art, the value of the current was generally set using a slightly higher rate of current increase for the sake of weld quality, so that there was the inconvenience that welding under such conditions promoted wear of the electrode tip and the frequency of dressing the electrode tip and tip exchange increased.
In addition, for the purpose of obtaining a proper rate of current increase, it was very difficult in the prior art to confirm, through experimentation, the extent of wear appearing on the electrode tip in relation to the number of successive welds performed by that tip.